The development of surgical techniques has made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
Stereotactic radiosurgery is such a minimally invasive treatment modality that allows delivery of a large single dose of radiation to a specific intracranial target while sparing surrounding tissue. Unlike conventional fractionated radiotherapy, stereotactic radiosurgery does not rely on, or exploit, the higher radiosensitivity of neoplastic lesions relative to normal brain (therapeutic ratio). Its selective destruction depends primarily on sharply focused high-dose radiation and a steep dose gradient away from the defined target. The biological effect is irreparable cellular damage and delayed vascular occlusion within the high-dose target volume. Because a therapeutic ratio is not required, traditionally radioresistant lesions can be treated. Because destructive doses are used, however, any normal structure included in the target volume is subject to damage.
One such non-invasive radiotherapy technique is so called LINAC (Linear Accelerator) radio therapy. In a LINAC radiotherapy system, a collimated x-ray beam is focused on a stereotactically identified intracranial target. In such an accelerator, electrons are accelerated to near light speed and are collided with a heavy metal, e.g. tungsten. The collision mainly produces heat but a small percentage of the energy is converted into highly energetic photons, which, because they are electrically produced, are called “x-rays”. The gantry of the LINAC rotates around the patient, producing an arc of radiation focused on the target. The couch in which the patient rests is then rotated in the horizontal plane, and another arc is performed. In this manner, multiple non-coplanar arcs of radiation intersect at the target volume and produce a high target dose, resulting in a minimal radiation affecting the surrounding brain. The x-rays are normally created by accelerating electrons to near light speed, and then colliding them with a heavy metal (e.g., tungsten). The collision mainly produces heat but a small percentage of the energy is converted to highly energetic protons, which are collimated and focused on the target.
Another system for non-invasive surgery is commercially available under the name of Leksell Gamma Knife®, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and is focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. Each of the sources provides a dose of gamma radiation which is insufficient to damage intervening tissue. However, tissue destruction occurs where the radiation beams from all radiation sources intersect or converge, causing the radiation to reach tissue-destructive levels. The point of convergence is hereinafter referred to as the “focus point”. Such a gamma radiation device is referred to and described in U.S. Pat. No. 4,780,898.
In the system, the head of a patient is immobilized in a stereotactic instrument which defines the location of the treatment volume in the head. Further, the patient is secured in a patient positioning system which moves the entire patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system.
Consequently, in radiotherapy systems, such as a LINAC system or a Leksell Gamma Knife® system, it is of a high importance that the positioning system which moves the patient so as to position the treatment volume in coincidence with the focus point of the radiation unit of the system is accurate and reliable. That is, the positioning system must be capable of positioning the treatment volume in coincidence with the focus point at a very high precision. Furthermore, this high precision must also be maintained over time.
A predetermined position of a positioning system in a radiation therapy system comprising a radiation therapy unit can be determined relative to a fixed radiation focus point of the radiation therapy unit by radiation measurements, e.g., using a phantom with radiation sensitive film provided in a certain position within the phantom. Another method is applying a radiation sensitive film on a tool adapted to be mounted in the positioning system, which tool is provided with reference marks such that it can be mounted in a defined position relative to the positioning system. According to a further method, a phantom with an ionization chamber provided in a certain position within the phantom is used. However, these indirect methods are time-consuming and inaccurate.
In accordance with a further method, a PN diode (or PN diodes) mounted on a measurement tool, providing an output that is substantially proportional to the detected radiation, is used to determine a predetermined position of the positioning system relative to a fixed radiation focus point. The signal from the diode is amplified and measured. The diode or diodes are scanned over the stationary focus point of the radiation unit of the radiation therapy system. Measurement values regarding the coordinates of the positioning system are collected or obtained where a gradient of the radiation is high, i.e. at the edges of the radiation curve.
However, the measurement tool must itself be calibrated, which is conducted in a master system. The master system has, in turn, been calibrated by means of film measurements. Hence, in order to verify the predetermined position of a positioning system of a radiation therapy system, a measurement tool that has been calibrated in another radiation therapy system, the master system, is used. Even if the master system has been calibrated using film measurements, a certain degree of inaccuracy will remain. Further, in order to calibrate the measurement tool, access to the master system is required, which may be a limited resource in terms of accessibility, and the calibration has to be performed by specially trained personnel.
Thus, there is a need for an efficient and reliable method for calibrating a measurement tool for measuring the radiation in a radiation system such as a radiation therapy system. Thereby, an efficient and reliable determination or verification of a predetermined position of the positioning system in a radiation therapy system comprising a radiation therapy unit relative to a fixed radiation focus point of the radiation therapy unit can be achieved.